Automatic intensity control for electronic displays

ABSTRACT

A pulsed waveform generation means with duty-cycle a function of ambient light intensity utilizes multivibrator means including a photo resistor to vary coupling time constant such that on-time half-cycle durations vary with ambient light. The waveform is usable in conjunction with bi-level logic means to apply power on a variable duty-cycle pulsed bias to indicator or annunciator lamps.

United States Patent [1 1 Steen 1 Sept. 17, 1974 AUTOMATIC INTENSITY CONTROL FOR ELECTRONIC DISPLAYS [75] Inventor: Carl E. Steen, Cedar Rapids, Iowa [73] Assignee: Collins Radio Company, Dallas,

Tex.

22 Filed: Oct.29, 1973 21 Appl. No.: 410,587

[52] US. Cl 307/311, 307/273, 328/2 [51] Int. Cl. H03k 17/00 [58] Field of Search 307/311, 273; 328/2, 207

[56] Y References Cited UNITED STATES PATENTS 3,128,412 4/1964 Abromaitis 307/311 3,294,982 12/1966 Russo 328/207 FOREIGN PATENTS OR APPLICATIONS 818,118 8/1959 Great Britain 307/311 1,175,272 8/1964 Germany 307/273 Primary Examiner-Rudolph V. Rolinec Assistant Examiner-B. P. Davis Attorney, Agent, or Firm-Richard W. Anderson; Robert J. Crawford [5 7] ABSTRACT A pulsed waveform generation means with duty-cycle a function of ambient light intensity utilizes multivibrator means including a photo resistor to vary coupling time constant such that on-time half-cycle durations vary with ambient light. The'waveform is usable in conjunction with bi-level logic means to apply power on a variable duty-cycle pulsed bias to indicator or annunciator lamps.

7 Claims, 10 Drawing Figures PAIENIEIISEPIHHM 3.836.797

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PATENTED 3. 836 .797

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G Q LFLFLILITI I l 30 I I 1 2i U U l L 1 I 1 27 26 7 ANNUNCIATOR F1 LOGIC ANNUNCIATOR "ON"LOeIO 5 3 SWITCHING LAMP 29 MEANs I 28 A 32 ILLUMINATION POWER PHOTO .1; (P/O 9) SOURCE REsIsTOR 1 2 30' 24 k g B bg gggg 3O LOGIC I INDICATO I SWITCHINGL- ILLUMINATION GENERATOR (FIGSI1,5.7.8) ILLLLHEHNQTION MEANS 23 MEANS AUTOMATIC INTENSITY CONTROL FOR ELECTRONIC DISPLAYS This invention relates generally to intensity control of incandescent lighting means more particularly to a means for controlling the intensity of incandescent lighting means as a function of ambient light level.

The present invention finds special usage in controlling intensity of indicator and annunciator lamps throughout the range of extremely bright to dark ambient lighting environments. The lighting of various cockpit indicators and annunciator lamps in aircraft, for example, is extremely critical in that, under bright ambient conditions, the intensity of the displays or annunciators must be sufiicient to provide contrast and readability while, under dark ambient conditions, the intensity of the indicator lamps must be appreciably less to prevent undue glare which would detract from an observers vision capabilities.

Means generally employed in aircraft instrument and annunciator lamp lighting are manual in nature. The observer manually adjusts, either electrically or mechanically, the desired intensity of various lighted indicators for the best level corresponding to the then existing ambient light condition. This manual adjustment is at best marginally effective in properly controlling illumination levels of indicators which are continually lighted. In the case of momentary or periodic lamp energization such as is associated with annunciators (such as marker beacon lamps and transponder reply lamps) it is extremely important that commanded annunciations are not of a light intensity which could momentarily blind the observer under dark ambient conditions and which, conversely, are of insufficient intensity to attract the observers attention under bright ambient conditions.

Means have been employed in the art to automatically control the intensity of lighting such that the illumination level of indicators and annunicators is adjusted for ambient light conditions without the necessity of manual control by the observer. Generally, these approaches employ photo resistors in a dimming arrangement to present a variable resistance in series with the lamp element to control the intensity of energization. However known systems generally do not provide a reduction in illumination power requirement attendant to a reduction in illumination level.

Accordingly, an object of the present invention is the provision for automatic intensity control of indicators or annunciator lamps in a manner which provides increased efficiency by operating on an ON-OFF switching of the lamp power source.

A further object of the present invention is the provision of an improved automatic intensity control for lamps which requires no manual adjustment, and requires minimal circuitry along with the advantages of low cost and inherent accuracy and stability.

Another object of the present invention is the provision of an automatic intensity control for indicating lamps based on a bi-level logic variable duty cycle waveform generation by means of which power may be switched to a lamp.

A still further object of the present invention is the provision for controlling the intensity of indicator and annunciator lamps as a direct function of ambient light level under control of a variable duty cycle waveform OFF control for indicator lamps byemploying a photo resistor in circuit with multivibrator time constant defining means to develop an output driving waveform the duty cycle of which decreases with decrease in ambient light level. The waveform may be utilized directly to control logic switching means for application of power toindicator lamps and may be logically combined with further logic associated with annunciator commands to control annunciator intensity level as a function of ambient light level.

These and other features and objects of the present invention will become apparent upon reading the following description with reference to the accompanying drawings in which:

FIG. 1 is a schematic representation of a waveform generation means to provide a variable duty cycle output waveform in accordance with the present invention;

FIG. 2 illustrates output waveforms of the generator of FIG. 1;

FIG. 3 is schematic representation of a monostable multivibrator in circuit with a photo resitor to provide an output pulse with duration defined by ambient light level;

FIG. 4 illustrates operational waveforms of the multivibrator of FIG. 3;

7 FIG. 5 is a preferred embodiment of a waveform generator in accordance with the present invention, employing a pair of crosscoupled monostable multivibrators;

FIG. 6 illustrates output waveforms of the circuitry of FIG. 5;

FIG. 7 is a schematic diagram employing a commercially available dual-mononstable multivibrator integrated circuit to arrive at a waveform generator as depicted in FIG. 5;

FIG. 8 is a schematic representation of a further waveform generating means in accordance with the present invention;

FIG. 9 illustrates output waveforms of the circuitry of FIG. 8; and

FIG. 10 is a functional diagram employing variable waveform generators of FIG. 1, 5, 7 or 8 to control indicator illumination means and annunciator lamps.

The waveform generating means of the present invention comprises astable multivibrator means with a photo reisitor in circuit with the multivibrator to control the time duration of successive half-cycles of the multivibrator output waveform. Complementary output waveforms are. available, either of which, in conjunction with appropriate control logic, may be utilized to switch an illumination means suchthat resulting illumination level is decreased as ambient light level decreases and conversely is increased as ambient light level increases.

A first embodiment of a waveform generator the out-' the mu tivibrator outputs 30 and 31, respectively designated Q and Q, are symmetrical square-waves. With different collector time constants, the duration of the individual portions of a complete output cycle are correspondingly different.

In accordance with the present invention, the time constant R,C is controlled as function of ambient light intensity by shunting resistor R, with a photo resitor 15. Operational waveforms depicted in FIG. 2 illustrate that the Q output 31 exhibits successively longer highlevel states as the ambient light impinging on photo resistor 15 decreases. Complementary output waveforms are generated and it is seen that should the Q output be used to energize a lamp during the low level state thereof, the duty cycle of the lamp energization level would decrease under decreased ambient light condi: tions. Conversely, should the high level output of the Q waveform be utilized to energize a lamp, the energization level would likewise decrease under decreased ambient light conditions. Thus, over a control range limited due to design considerations of recovery time, transit time, etc., the astable multivibrator of FIG. 1 with the time constant variation effected by photo resistor 15 may serve as a waveform generator to provide a logic output which may be used to control lamp energization.

Because of instability factors and control range limitation associated with monostable multivibrator operation, coupled with the fact that the monostable multivibrator may exhibit unreliable tum-on characteristics when energized, a preferred waveform generator in accordance with the present invention is based upon the employment of a pair of monostable multivibrators with outputs cross-coupled. As will be described, one of the pair of monostable multivibrators may have its output pulse time duration defined as a function of ambient light to arrive at the desired duty-cycle output waveform for lamp energization control. Because monostable multivibrators are rapidly retriggerable, the preferred waveform generator embodiment exhibits reliable turn-on characteristics and offers a wider range of control of the output waveform duty cycle.

FIG. 3 illustrates a conventional monostable multivibrator comprised of transistors 12 and 13 with the duration of the output pulses being controlled as a function of ambient temperature by the inclusion of photo resistor 15 to vary the pulse-width defining RC'time constant. Under quiescent conditions, transistor 13 is conductive and transistor 12 cut-off, such that the output is low (See waveform of FIG. 4). Upon application of a positive triggering impulse 14 to the base of transistor 12 or, alternatively, a negative triggering impulse 14 to the collector of transistor 12, transistor 12 is triggered into conduction and, due to the crosscoupling, transistor 13 is cut-off. The amount of time that transistor 13 remains cut-off is dependent upon the RC time constant. The base voltage on transitor 13 rises exponentially towards the positive voltage supply as C charges through R. Thus transitor 13 comes out of cut-off when the base voltage on transistor 13 rises to a predetermined design level at which transitor 13 once again goes into conduction, forcing transistor 12 into cut-off. Transistor 13 develops a 0 output exhibiting successive high level states of longer time duration as the ambient light impinging on photo resistor 15 decreases with an attendant increase in the RC time constant.

FIG. 4 illustrates the Q and?) outputs. In response to successive triggering pulses l4 and 14' the Q output rises to a high level for a predetennined time which is controlled by the ambient light level and then returns to its quiescent low state until the time occurrence of the succeeding trigger pulse. It is noted that the width of the successive Q output pulses increases in proportion to decrease inambient light level and thus a monostable multivibrator may be utilized to provide, in response to a triggering input thereto an output pulse with time duration a function of the then existing ambient light level. Further, if trigger pulses 14 or 14 are of fixed periodicity, the duty cycle of the output waveform varies as a direct function of ambient light level.

The preferred waveform generator of the present invention consists in the combination of two monostable multivibrators into a single circuit, with the monostable multivibrators cross-coupled so as to collectively function as an astable multivibrator. One of the monostable multivibrators is provided with a fixed RC time constant so as to provide, in response to a triggering input thereto, a'fixed duration output'pulse which may be used to establish triggering impulses of fixed delay for application to the other of the multivibrators which is provided with an RC time constant which varies as a function ambient light intensity by the inclusion of photo resistor 15 in shunt with the resistance compo nent which defines the RC time constant thereof.

With reference to FIG. 5, first and second multivibrators MV-l and MV-2 are illustrated with crosscoupling between outputs such that, in operation, the termination of the pulse output from MV-l is utilized to initiate (trigger) the pulse output of MV-2, and vice versa. Multivibrator MV-l, comprising transistors 16 and 17, produces, in response to each triggering input to transistor 16, an output pulse of time duration defined by the time constant R,C Multivibrator MV-2, comprising transistors 18 and 19, produces, in response to each tiggering input to transistor 18, an output pulse with time duration defined by the time constant R=C, which in turn is a function of ambient light level.

In operation, the termination of successive fixedduration output pulses of MV-l are utilized to define the initiation of an output pulse from MV-2, and vice versa. FIG. 5 illustrates base triggering of the quiggcently" nonconductive transistors 16 and 18,-The Q, output from transistor 16 of MV-l is applied to the base of the quiescently nonconductive transitor 18 of MV-2 to effect triggering, while the 0, output from transistor 16 of MV-l is applied to the base of the quiescently nonconductive transistor 18 of MV-2 to effect triggering. Triggering is here effected by the application of a positive-going signal to the respective base elements.

Operational waveforms of the waveform generator of FIG. 5 are depicted in FIG. 6, and illustrate the variation in output waveform duty cyle from bright ambient to dark ambient light conditions. Waveform 61, the output 30 from FIG. 5, is comprised of sucessive likeduration positive-going pulse corresponding to the fixed pulse width fo MV-l. Waveform 61 illustrates successively longer duration positive going output pulses as defir d by the variable pulse width of MV-2. Since rises in 0, correspond to the termination of successive MV-2 Q, pulses, and are coupled to trigger transitor 16 of MV-l, the termination of successive output pulses Q define the initiation of successive output pulses from transistor MV-l. Two pairs of waveforms are therefore available. The first pair, designated output 30-30', comprises the respective like Q and Q outputs, while the inverse pair 31-31" comprises the respective like Q and Q outputs.

Either of outputs 30-30 may be utilized to energize a lamp during the high-level state thereof such that illumination decreases as ambient light level decreases. Correspondingly, either of outputs 31-31 may be utilized to energize a lamp during the low level state thereof with the same result.

Waveform generator circuitry of FIG. 5 may be economically implemented using a commercially available Type 74123 dual monostable multivibrator integrated circuit, as depicted in FIG. 7. The integrated circuit comprises blocks 20' and 21 which, in conjunction with illustrated external circuitry as shown arrive at a waveform generator similar to that schematically illustrated in FIG. 5.

FIG. 8 illustrates a further waveform generator embodiment employing a pair of monostable multivibrators to arrive at the desired duty cycle output waveform. Multivibrators MV-l and MV-2, like those in FIG. 5, are cross-coupled to effect collector triggering of the quiescently nonconductive transistors 16 and 18. The output Q of transistor 19 of MV-2 is coupled to the collector of transistor 16 of MV-l such that successive falls in the waveform Q trigger transistor 16 into conduction. The output Q from the collector of transistor 17 of MV-l is coupled to the collector of transistor 18 of MV-2 to similiarly effect triggering on negative-going falls. In this embodiment, likewaveforms Q and Q comprise a common output 30.

Output waveforms 30 and 31 of FIG. 8 are depicted in FIG. 9. Successive negative-going edges of Q, pulses determine the initiation of successive positive-going 0 pulses, and vice versa. Again, either of the outputs 30 or 31 may be utilized with appropriate logic circuitry to control the intensity of lamps in direct proportion to ambient light level since the photo resistor in circuit with multivibrator MV-2 decreases the output waveform duty cycle with decrease in impinging ambient light.

When the fixed duration alternate half-cycles of the above-discussed waveforms are utilized to determine the on-time of a lamp to be energized (as determined by MV-l), or the off-time of the lamp to be energized is determined by the successive half-cycles determined by the pulse width of the multivibrator MV-2, a wide range of illumination control is obtainable. For example, in an embodiment which was caused to be constructed, a type CL907N photo resistor was utilized in parallel with a resistance of 47,000 ohms. Under bright ambient conditions the parallel resistance was approximately 500 ohms, while under dark ambient conditions the parallel resistance was essentially 47,000 ohms. The ratio of on-time to off-time was variable from 1/10 to 10 which varied the lamp duty factor from l/ 1.1 to l l 1. This on-off ratio could obviously have been modified by changing the value of either or both of the time constant determining fixed resistors R and R FIG. 10 functionally represents the incorporation of the previously described waveform generator circuitries of FIGS. 1, 5, 7 and 8 to control the intensity of both momentary annuciator lamps and indicator illumination means. The variableduty cycle waveform generator 9, which might comprise the circuitries of FIGS. 1-, 5, 7 or 8, isdepicted with the photo resistor 15 placed in proximity with the location of an annunciator lamp v 29 and an indicator illumination means 24. The illumination level controlling waveform designated 30, 30' is applied from the waveform generator 9 to a first logic switching means 22. In the exampled usage of the output waveform 30, 30', the assumption is made that high level logic is instrumental in causing logic switching means 22 to apply power from illumination power source 32 as a pulsed input 23 to indicator illumination means 24.

Because the waveform generator provides a bi-level logic output, the output may be readily used to control the illumination intensity of annunciator lamps in accordance with ambient light intensity. Thus, FIG. 10 illustrates the output 30, 30' being further applied as a first input to an AND gate 26 to which a high-level annunciator-on logic 25 is applied as a second input. The output from AND gate 25 would thus comprise the waveform generator output 30, 30 only when gated by the annunciator-on logic 25. The AND gate 25 output may be applied to a further logic switching means 27 to switch the illumination power source 32 as a pulsed input 28 to an annunciator lamp 29. Obviously, inverse logic could be employed, utilizing the complementary outputs obtainable from the waveform generator 9 and appropriate inverse logic in the control system.

The present invention is thus seen to provide relatively simple and economical means for controlling lamp engergization as a direct function of ambient light intensity. Because the control is effected by a logic waveform, a relatively low-level waveform generator circuitry may be utilized to control relatively high power illumination requirements. An inherent increase in control efficency is provided by the on-off switching of illumination power since the control system operates on absolute utilization of power by the indicating lamp rather than upon a dimming basis where power not utilized for illumination is nontheless consumed.

Although the present invention has been described with respect to particular embodiments thereof, it is not to be limited, as changes might be made therein which fall within the scope of the invention as defined in the appended claims.

Wherein I claim:

1. An intensity control circuit for controlling the illumination intensity of a lighted indicating means as a direct function of ambient light intensity, comprising a multivibrator, means for controlling the time duration of first alternate half-cycles of the output from said multivibrator as a direct function of ambient light intensity, said multivibrator comprising an RC time constant means the resistance parameter of which comprises a photo resistor connected in shunt with a fixed resistance member, said RC time constant means defining the time duration of said first alternate half-cycles, and means for energizing said lighted indicating means on a duty cycle basis as defined by second alternate half-cycles of the output from said multivibrator.

2. Waveform generating means comprising first and second monostable multivibrators each, in responseto a triggering input thereto, developing an output pulse of a predetermined time duration, means coupling the output of each monostable multivibrator as a triggering input to the other of said monostable multivibrators,

the output of one of said monostable multivibrators comprising the output of said waveform generating means and having a duty cycle on-time defined by the design on-time of one of said monostable multivibrators and a duty cycle off-time defined by the design ontime of the other one of said monostable multivibrators, the on-time of one of said monostable multivibrators being selectively variable, and control means whereby said selectively variable on-time is directly proportional to ambient light intensity.

3. Waveform generating means as defined in claim 2 wherein said one of said monostable multivibrators comprises an RC time constant means defining the ontime thereof, and means for controlling the resistance parameter of said RC time constant means as a direct function of ambient light intensity.

4. Waveform generating means as defined in claim 3 wherein said resistance parameter comprises a photo resistor shunting a fixed resistance member.

5. An intensity control for controlling the intensity of a lighted indicating means as a direct function of ambient light intensity, comprising waveform generating means, said waveform generating means comprising first and second monostable multivibrators each, in response to a triggering input thereto, developing an output pulse having a predetermined time duration, means coupling the output of said monostable multivibrators, multivibrator as a triggering input to the other of said monostable multivibrators, the output of one of said monostable multivibrators comprising the output of said waveform generating means and having a duty cycle on-time defined by the design on-time of one of said monostable multivibrators and a duty cycle offtime defined by the design on-time of the other of said multivibrators, means for controlling the on-time of one of said monostable multivibrators as a direct function of ambient light intensity, and means for energizing said lighted indicating means on a duty cycle basis as defined by the on-time of the other one of said monostable multivibrators.

6. An intensity control circuit as defined in claim 5 where said one of said monostable multivibrators comprises an RC time contant means defining the on-time thereof and means for controlling the resistance parameter of said RC time constant means as a direct function of ambient light intensity.

7. An intensity control circuit as defined in claim 6 wherein said resistance parameter comprises a photo resistor shunting fixed resistance member.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION- PatntNo. 3,836,797 ee'eee September 11, 1974 Inventor(s) Car 1 E. Steen 7 It is certified that error appears in the ahore-identified patent" and that said Letters Patent are hereby corrected as shown below IN THE SPECIFICATION: V I I Column 1 JineLS, after "means," insert -,-and CoIumn 3 line 7 after "as" inser t -'-a--. Column 4, 1 ine27', after "function" vinsertj -oiv Tine 62, change""1ikeduration" to- -li ke-duration- (l2o1umn 8; line 2, change "said" to ,--each--;' I

delete "multifvibrators Signed and sealed this 24th day of December 2.974.

(SEAL) Attest: V

McCOY M. GIBSON JR. I I c'. MARSHALL 'DANN Attesting Officer v Commissioner of Patents I Q I I USCOMM-DC 60376-P69 ".5. GDVIINIINT 'RINTIIIG OFFICE 2 I" O-lil-Sll. 

1. An intensity control circuit for controlling the illumination intensity of a lighted indicating means as a direct function of ambient light intensity, comprising a multivibrator, means for controlling the time duration of first alternate half-cycles of the output from said multivibrator as a direct function of ambient light intensity, said multivibrator comprising an RC time constant means the resistance parameter of which comprises a photo resistor connected in shunt with a fixed resistance member, said RC time constant means defining the time duration of said first alternate half-cycles, and means for energizing said lighted indicating means on a duty cycle basis as defined by second alternate half-cycles of the output from said multivibrator.
 2. Waveform generating means comprising first and second monostable multivibrators each, in response to a triggering input thereto, developing an output pulse of a predetermined time duration, means coupling the output of each monostable multivibrator as a triggering input to the other of said monostable multivibrators, the output of one of said monostable multivibrators comprising the output of said waveform generating means and having a duty cycle on-time defined by the design on-time of one of said monostable multivibrators and a duty cycle off-time defined by the design on-time of the other one of said monostable multivibrators, the on-time of one of said monostable multivibrators being selectively variable, and control means whereby said selectively variable on-time is directly proportional to ambient light intensity.
 3. Waveform generating means as defined in claim 2 wherein said one of said monostable multivibrators comprises an RC time constant means defining the on-time thereof, and means for controlling the resistance parameter of said RC time constant means as a direct function of ambient light intensity.
 4. Waveform generating means as defined in claim 3 wherein said resistance parameter comprises a photo resistor shunting a fixed resistance member.
 5. An intensity control for controlling the intensity of a lighted indicating means as a direct function of ambient light intensity, comprising waveform generating means, said waveform generating means comprising first and second monostable multivibrators each, in response to a triggering input thereto, developing an output pulse having a predetermined time duration, means coupling the output of said monostable multivibrators, multivibrator as a triggering input to the other of said monostable multivibrators, the output of one of said monostable multivibrators comprising the output of said waveform generating means and having a duty cycle on-time defined by the design on-time of one of said monostable multivibrators and a duty cycle off-time defined by the design on-time of the other of said multivibrators, means for controlling the on-time of one of said monostable multivibrators as a direct function of ambient light intensity, and means for energizing said lighted indicating means on a duty cycle basis as defined by the on-time of the other one of said monostable multivibrators.
 6. An intensity control circuit as defined in claim 5 where said one of said monostable multivibrators comprises an RC time contant means defining the on-time thereof and means for controlling the resistance parameter of said RC time constant means as A direct function of ambient light intensity.
 7. An intensity control circuit as defined in claim 6 wherein said resistance parameter comprises a photo resistor shunting fixed resistance member. 